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Hydrate Equilibrium Data for CO2+N-2 System in the Presence of Tetra-n-butylammonium Fluoride (TBAF) and Mixture of TBAF and Cyclopentane (CP)

Tzirakis, Fragkiskos; Stringari, Paolo; Coquelet, Christophe; von Solms, Nicolas; Kontogeorgis,Georgios

Published in:Journal of Chemical and Engineering Data

Link to article, DOI:10.1021/acs.jced.5b00942

Publication date:2016

Document VersionPeer reviewed version

Link back to DTU Orbit

Citation (APA):Tzirakis, F., Stringari, P., Coquelet, C., von Solms, N., & Kontogeorgis, G. (2016). Hydrate Equilibrium Data forCO2+N-2 System in the Presence of Tetra-n-butylammonium Fluoride (TBAF) and Mixture of TBAF andCyclopentane (CP). Journal of Chemical and Engineering Data, 61(2), 1007-1011.https://doi.org/10.1021/acs.jced.5b00942

1

Hydrate equilibrium data for CO2+N2 system in the

presence of Tetra-n-butylammonium fluoride (TBAF) and

mixture of TBAF and cyclopentane (CP)

Fragkiskos Tzirakis*,1, Paolo Stringari

2, Christophe Coquelet

2, Nicolas von Solms

1 and

Georgios Kontogeorgis1

1Center for Energy Resources Engineering, Department of Chemical and Biochemical Engineering,

Technical University of Denmark, DK-2800 Kgs. Lyngby, Denmark 2Mines ParisTech PSL Research Université CTP-Centre of Thermodynamic of Processes 35 Rue

Saint Honoré, 77305 Fontainebleau, France Keywords: TBAF, CP, promotion, hydrates

Abstract

Hydrates can be used for CO2 capture from flue gases (hydrate crystallization). In this

work hydrate equilibrium data were measured and compared with literature data. The

isochoric method was used to determine the gas hydrate dissociation points.

Different CO2+N2 gas mixtures were used with presence of promoters such as tetra-

n-butylammonium fluoride (TBAF) and mixtures of TBAF and cyclopentane (CP). The

key novel aspect of this work is the use of combination of promoters, TBAF and CP,

which under certain conditions induced further pressure reduction in comparison to

pure TBAF results. Concerning experiments with pure promoter, there is good

agreement between our results and literature results for different gas mixtures and

promoter concentrations.

Introduction

The need of this work data lies in the development of novel separation methods for

CO2 from flue gases. There are three approaches for CO2 separation: pre-

combustion decarbonization, oxyfuel combustion and post combustion CO2

separation. For post combustion CO2 separation from flue gas, four different

approaches are mainly used; chemical and physical absorption, solid physical

adsorption – pressure swing and temperature swing adsorption, low temperature

distillation (cryogenic separation) and membrane separation. Moreover, hydrate

crystallization could become in the future an economically viable alternative process

for post combustion CO2 separation from flue gas in oil and gas processing1.

2

Gas hydrates are crystalline solids composed of water and gas. The major contrast

with respect to ice is that ice forms as a pure component, while hydrates will not form

without guests of the proper size. Furthermore, hydrates can form at temperatures

higher than 273 Κ. The gas molecules (guests) are captured in water cavities (host)

that consists of hydrogen-bonded water molecules. Gas molecules which typically

form hydrates are methane, ethane, propane and carbon dioxide. All common natural

gas hydrates belong to the three crystal structures, cubic structure I (sI), cubic

structure II (sII), or hexagonal structure H (sH). In semiclathrate hydrates, the water

molecules together with anions such as Br− and F− build a polyhedral host set of the

cages in which the tetra-nbutylammonium cations (TBA+) are attached as guest

molecules.2 The main difference between semiclathrate and crystal gas hydrates, is

that in the first one there are ionic interactions with the host molecules (guest

molecules can both form part of the host lattice and occupy cages of the structure)

while in the latter the guest molecules are not bonded physically to the host water

lattices. Many studies on semiclathrate hydrates have recently reported the capture

of CO2 from flue and fuel gas mixtures.3-5

Promoters (or hydrate formers) are mainly organic compounds classified in two

groups: thermodynamic and kinetic. The first ones extend the hydrate formation

region in a P-T diagram. Kinetic promoters enhance the hydrate formation rate.

Quaternary ammonium salts like tetra-nbutylammonium bromide (TBAB) and fluoride

(TBAF) form ionic semiclathrate hydrates with water molecules at atmospheric

pressure and they are used as thermodynamic promoters.2

From industrial point of view, the separation steps are the most costly while the

design of other required equipment is in general simple.6 So, the price of the

promoters needed to reduce the pressure and increase the temperature is where

economic studies should mainly focus on. The industry will be interested in such

investments whenever the environmental regulations are rigid and when the natural

gas reserves tend to reach their half-lives.

In general, for process development the operating temperature, minimum pressure

for hydrate formation, the rate of hydrate formation and the separation efficiency

should be established.7 TBAF exhibits greater pressure reduction than TBAB.8

According to literature9, the time to reach hydrate equilibrium with TBAF is about one-

half of that with TBAB and one-fourth of that with THF. TBAF space velocity and CO2

separation factor are also higher than TBAB´s. In addition, the highest stabilization

3

effect was observed at the stoichiometric concentration of pure TBAF semiclathrate,

which is 3.3 mol %.10 In the contrary, the gas uptake is higher for TBAB than for

TBAF.11

Cyclopentane is already used as promoter in many studies.12-15 Cyclopentane

emulsion is formed at higher than 8.99 mol %12 concentrations. The emulsion lowers

the promoter΄s pressure reduction efficiency. The cyclopentane/water emulsion has

higher hydrate formation rate than that with cyclopentane because of the larger gas

and liquid contact area that controls hydrate formation rate16. In addition, the CO2

selectivity in hydrates using cyclopentane is improved in comparison to the system

without promoter and the equilibrium pressure is drastically reduced. However, the

gas storage capacity is also reduced11. Even though cyclopentane is a very good

promoter, but it stabilizes the cavities in such a way that it prevents the complete

occupation of the remaining cavities by gas molecules.

In this study hydrate equilibrium results of thermodynamic promoters (TBAF and

TBAF+CP) are presented and compared with existing literature. The purpose of this

study is the generation of data for gas mixtures of CO2 and N2 with low CO2 content

−which simulates flue gas composition of post combustion power plant− as well as

the examination whether CP induces promotion effect together with TBAF for

CO2+N2 gas mixtures. A preliminary literature study showed lack of data concerning

this system.

Experimental

Experimental section

Materials. The chemicals used are shown in Table 1. The gas cylinders of CO2 and

N2 gases used in this work were supplied by Air Liquide. The molar fractions of CO2

in N2 was 0.48 mol % which is a low CO2 concentration used for oil and gas

processing16. TBAF solutions with mass fractions of 0.032, 0.05 and 0.10 were

prepared by the gravimetric method using an accurate analytical balance (Mettler,

AT200), with mass accuracy of ±0.0001 g. Double-distilled and deionized water from

Direct-Q5 Ultrapure Water Systems (MilliporeTM), was used in all experiments.

Cyclopentane at concentration of 5 vol. % was added after in 5 mol % and 10 mol %

TBAF solutions with use of proper syringe.

4

Table 1. Chemicals used in this study

Chemical Name

Abbreviation

CAS-number

Purity Phase Supplier

Carbon dioxide

CO2 124-38-9 99.998 (Vol.

%)

gas Air Liquide

Nitrogen CO-free

N2 7727-37-9 99.999 (Vol.

%)

gas Air Liquide

Tetra-n-butylammoniu

m fluoride

TBAF 429-41-4 75 (wt) % in

H2O

liquid Sigma Aldrich

Cyclopentane CP 287-92-3 ≥98 % (wt %) liquid Acros

Organics

Water H2O 7732-18-5 deionised liquid -

Apparatus. The equipment is identical to the one used in previous study17. Isochoric

temperature trace method is applied. Two gas cylinders were used, one for nitrogen

and one for CO2 and N2. The nitrogen cylinder is used for cleaning the equilibrium

cell and the CO2 and N2 gas cylinder contains the gas mixture of this study. The

equilibrium cell is immersed in a water bath for controlling the temperature. The

equilibrium cell temperature is controlled using a thermostatic water bath (LAUDA

PROLine RP3530). Two platinum temperature probes (Pt100, JM6081) inserted in

the cell interior –at the top (gas phase) and the bottom (liquid phase)– were used to

measure the temperature inside the equilibrium cell. The temperature accuracy,

determined by calibration of the respective transducers, is estimated to be ±0.02 K

after careful calibration against reference platinum probe (TINSLEY Precision

Instruments). The pressure in equilibrium cell is measured using a UNIK 5000 GE

absolute pressure transducer with an accuracy of ±0.0015 MPa after careful

calibration against dead weigh balance. The standard uncertainty of gas mixture

composition y for CO2/N2 (0.48/99.52) gas mixture, measured from Gas

Chromatograph, for CO2 is 0.58 % and for N2 0.70 %.

The equilibrium cell is made of the 316 stainless steel; its maximum working pressure

and its inner volume is 40 MPa and 125 mL, respectively. A motor-driven turbine

agitation system (Top Industrie, France) enables to stir the cell contents at a speed

up to 1200 rpm to increase the fluids contact and enhance water conversion into

hydrate. The data acquisition units (Agilent 34970A, HP 34970A) were connected

5

with a personal computer. Continuous recording of pressures and temperatures

allows detecting any subtle changes in the system and true equilibrium conditions.

Experimental procedure. The procedure is similar to the one used in previous

study17. After evacuation of the equilibrium cell using the vacuum pump (Oerlikon

leybold vacuum, Trivac D2.5E), 25-40 ml of promoter solution (TBAF, TBAF+CP)

−that is about 25-30 vol. % of the equilibrium cell− was subsequently partially filled in

the equilibrium cell. Then, the gas mixture was introduced in the equilibrium cell from

the cylinder. In the end, several P-T equilibrium data were obtained from each

experimental run and eventually a P-T diagram is created following temperature-

pressure trace method.

Results and discussion

The TBAF results for CO2+N2 gas mixture global concentrations are summarized in

Figure 1. The results are compared with literature data. At first, for comparison

purposes, the unpromoted system N2 is reported18. In general, it is observed good

agreement of our results with the literature data for similar systems of 5% and 10 wt

% TBAF solutions which correspond to 0.36% and 0.76 mol % respectively. For 3 wt

% TBAF solution there are no systems in literature.

For clarity reasons, the systems are presented by two numbers in brackets. The first

number denotes the mol fraction of CO2 in CO2+N2 gas mixture cylinder and the

second one denotes the promoter concentration expressed in mol %. Black markers

connected with trendlines correspond to results of this work. From Gibbs phase rule,

the parameters that suggest where the equilibrium lines should be located are the

gas mixture concentration, the promoter concentration in aqueous solution and the

water-to-gas ratio (mol/mol). For simplicity reasons and also owning to the fact that

gas-to-liquid ratio is not always mentioned in literature, it was omitted from this study.

The results for 3.2%, 5% and 10 wt % TBAF, which correspond to x=0.23%, x=0.36%

and x=0.76 mol % respectively, seem well placed in the Figure 1. The gas mixture,

which is 99.52% mol N2, produced steeper results than with higher CO2

concentration. In addition 0.36 mol % TBAF are in good agreement with similar

solutions from literature19-21 even though the last correspond to pure CO2 results.

6

Figure 1. Hydrate dissociation points for different systems using TBAF as promoter with CO2 (1) + N2

(2) (0.48/99.52) gas mixture. All values are expressed in mol %. Black straight line connects the results of this work. Symbols correspond to experimental data: ◆ , x1=0.0, xTBAF=0.0, ref. 18; ●, x1=0.48, xTBAF=0.23, this work; ▲, x1=0.48, xTBAF=0.36, this work; ◆, x1=0.0, xTBAF=0.36, ref. 19; X,

x1=30, xTBAF=0.68, ref. 20; ▀, x1=0.48, xTBAF=0.76, this work; —, x1=0.0, xTBAF=0.76, ref. 21.

Finally in Figure 2 all hydrate dissociation points of this work are presented. The use

of CP for 0.76 mol % TBAF has no effect in the results while for 0.36 mol % TBAF the

behavior is mixed. For low pressures (<30 bar), inhibition effect is observed while for

higher pressures (>30 bar) promotion effect appears. An explanation for this behavior

could be that at low pressures CP acts as inhibitor, due to its water insolubility,

obstructing the hydrate formation. At higher pressures, the kinetic phenomena play

more important role, so CP contributes positively to the construction of hydrates.

7

Figure 2. Hydrate dissociation points for different systems using TBAF (1) and CP (2) as promoter

with CO2 + N2 (0.48/99.52) gas mixture. CP concentration is given in volume fraction. All other values are expressed in mol %. Black straight line connects the results of this work. Symbols correspond to

experimental data: ●: x1=0.23, x2=0.0, this work; Ж: x1=0.36, x2=0.05, this work; ▲: x1=0.36, x2=0.0,

this work; ▀: x1=0.76, x2=0.05, this work; X: x1=0.76, x2=0.0, this work.

Consistency of experimental results

For data treatment, Clausius–Clapeyron method is applied, eq. 1. Clausius-

Clapeyron equation estimates the vapor pressures of liquids or solids.

𝑑𝑙𝑛(𝑃)

𝑑(1

𝑇)=

−𝛥𝐻𝑑𝑖𝑠

𝑍⋅𝑅 (1)

where ΔHdis is the apparent dissociation enthalpy of the hydrate phase, Z is the

compressibility factor and R is the gas constant. Lee-Kesler-Plöcker (LKP) Equation

of State (EoS)22 is applied for estimation of Z as a function of T and P using binary

8

interaction parameter κij=1.11. It is assumed very low solubility of the gas in the liquid

and, thus, no changes in the gas composition.

In this study, the interest lies on the fact that for small changes of dissociation

temperature, the Clausius–Clapeyron equation should predict identical (or similar)

ΔHdis values. The ΔHdiss. as a function of dissociation temperature shows the

goodness of fit (which is shown by the Coefficient of determination (R2)). Coefficient

of determination (R2) is used to show how well fit the ΔΗ values (kJ/mol) in the

straight line of ΔΗ(Τ). Τable 2 presents the data treatment for TBAF results of this

work and from literature.

Table 2. Coefficient of determination (R2) of ΔΗdiss. (kJ/mol) in terms of temperature including TBAF literature

Promoter concentration (mol %)

CO2 global concentration of feed in CO2+N2 gas

mixture (mol %)

Coefficient of determination

(R2)

Literature

TBAF+CP

0.36 0.05 0.995 this work

0.76 0.05 0.864 this work

TBAF

0.23 0.05 0.997 this work

0.36 0.05 0.991 this work

0.76 0.05 0.993 this work

0.29 100 0.285 Li et al.23

0.62 100 0.984 Li et al.23

0.14 100 0.986 Mohammadi et al.19

0.36 100 0.935 Mohammadi et al.19

1.20 100 0.734 Mohammadi et al.19

0.80 100 0.985 Lee at al.10

3.00 100 0.938 Lee at al.10

3.30 100 0.958 Lee at al.10

5.30 100 0.926 Lee at al.10

0.36 30.0 0.997 Sfaxi et al.20

0.68 30.0 0.988 Sfaxi et al.20

0.00 0.0 0.928 Van Cleeff and Diepen18

0.36 0.0 0.841 Mohammadi et al.19

1.20 0.0 0.922 Mohammadi et al.19

0.76 0.0 0.504 Lee at al.21

1.69 0.0 0.627 Lee at al.21

3.43 0.0 0.812 Lee at al.21

5.33 0.0 0.588 Lee at al.21

The results of this work are very good (R2>0.90) except for the system of 0.72 mol %

of TBAF+CP mixture. The system of 0.29 mol % in pure CO2 gas of Li et al.23 and 1.2

mol % in pure CO2 gas, 0.36 mol % in pure N2 gas of Mohammadi et al.19 as well as

9

the systems of Lee at al.21 are problematic (R2<0.90). The rest systems from

literature seem to be very good.

Experimental Uncertainties

The experimental uncertainties are presented in Tables 3 and 4. The standard

temperature and pressure uncertainties are proved to be satisfactory.

Table 3. Hydrate equilibrium point for TBAF solution with standard temperature and pressure uncertainties, U(T) and U(P).

T/K P/MPa TBAF concentration/ wt

% (mol %) U (T)/K U(P)/MPa

293.18 2.03 10 (0.76%) 0.057 0.0021 293.61 3.55 10 (0.76%) 0.035 0.0019 293.91 5.01 10 (0.76%) 0.030 0.0019 294.14 6.42 10 (0.76%) 0.025 0.0018 294.30 7.91 10 (0.76%) 0.049 0.0029 287.05 0.97 5 (0.36%) 0.023 0.0017 287.55 2.55 5 (0.36%) 0.023 0.0017 288.20 4.76 5 (0.36%) 0.033 0.0018 288.93 7.25 5 (0.36%) 0.028 0.0019 280.99 1.02 3.2 (0.23%) 0.023 0.0017 281.61 2.48 3.2 (0.23%) 0.023 0.0017 282.23 4.05 3.2 (0.23%) 0.023 0.0017 282.88 6.69 3.2 (0.23%) 0.023 0.0017

TBAF – wt% (mol %) +5 vol. % CP

292.64 0.97 10 (0.76%) 0.023 0.0017

293.34 2.57 10 (0.76%) 0.023 0.0017 293.91 4.77 10 (0.76%) 0.023 0.0017 294.05 7.14 10 (0.76%) 0.023 0.0017 286.29 1.28 5 (0.36%) 0.023 0.0017 287.78 2.99 5 (0.36%) 0.023 0.0017 288.44 4.73 5 (0.36%) 0.023 0.0017 289.62 6.91 5 (0.36%) 0.023 0.0017

Gas mixture composition standard uncertainty for CO2/N2 (0.48/99.52) gas mixture is

U(yCO2) = 0.58 % and U(yN2) = 0.70 %.

Table 4. Gas molar composition and gas inserted in equilibrium cell standard

uncertainty U(ngas) for every hydrate equilibrium point

CO2

inserted/

N2

inserted/

H2O

inserted/

Promoter

inserted/ U(ngas)/ % Promoter concentration/ wt %

10

mol mol mol mol

0.000199 0.0647 1.74 0.01276 1.94 10% TBAF

0.000218 0.0711 1.74 0.01276 1.85 10% TBAF

0.000225 0.0732 1.74 0.01276 1.83 10% TBAF

0.000231 0.0753 1.74 0.01276 1.80 10% TBAF

0.000397 0.1293 1.74 0.01276 1.78 10% TBAF

0.000419 0.1364 1.94 0.00674 1.67 5% TBAF

0.000227 0.0739 1.94 0.00674 2.02 5% TBAF

0.000316 0.1030 1.94 0.00674 1.72 5% TBAF

0.000318 0.1037 1.94 0.00674 1.72 5% TBAF

0.000111 0.0362 2.03 0.03318 2.25 3.2% TBAF

0.000181 0.0590 2.03 0.03318 1.76 3.2% TBAF

0.000177 0.0576 2.03 0.03318 2.72 3.2% TBAF

0.000384 0.1251 2.03 0.03318 1.99 3.2% TBAF

0.000161 0.0526 2.03 0.03318 2.90 3.2% TBAF

0.000212 0.0689 1.26 0.02430 1.82 10% TBAF+CP (5 vol. %)

0.000216 0.0703 1.26 0.02430 1.81 10% TBAF+CP (5 vol. %)

0.000382 0.1243 1.26 0.02430 1.77 10% TBAF+CP (5 vol. %)

0.000456 0.1485 1.26 0.02430 1.73 10% TBAF+CP (5 vol. %)

0.000157 0.0512 1.31 0.01902 1.96 5% TBAF+CP (5 vol. %)

0.000297 0.0966 1.31 0.01902 1.72 5% TBAF+CP (5 vol. %)

0.000268 0.0874 1.31 0.01902 1.74 5% TBAF+CP (5 vol. %)

0.000395 0.1286 1.31 0.01902 1.68 5% TBAF+CP (5 vol. %)

Conclusions

Hydrate equilibrium points for CO2 and N2 were measured with the use of tetra-n-

butylammonium bromide (TBAF) and mixture of TBAF with CP as promoters. The

use of TBAF concentration (0.36 mol %) and CP (5 vol. %) revealed promotion effect

at pressures higher than 3.5 ΜPa. On the contrary, the results have shown that the

simultaneous use of TBAF (0.76 mol %) with CP (5 vol. %) did not have any impact

on thermodynamic equilibrium. The equilibrium results are more dependent on TBAF

concentration rather than gas mixture concentration as the comparison with literature

data revealed. The consistency analysis has shown that the results of this study are

satisfactory.

Author information

Corresponding author

*Tel.: +45-45252821. E-mail: frtz@kt.dtu.dk

11

Funding

The hydrate equilibrium data were measured at the Centre of Thermodynamics of

Processes of MINES ParisTech (France) as part of a collaborative project funded by

the Danish Technical Research Council (FTP project: CO2 hydrates — Challenges

and possibilities, Project nr. 50868). The authors wish to thank Ing. Alain Valtz for his

technical assistance and fruitful discussion.

Notes The authors declare no competing financial interest.

Acknowledgements

The hydrate equilibrium data were measured at the Centre Thermodynamic of

Processes of MINES ParisTech (France) as part of a collaborative project funded by

the Danish Technical Research Council (FTP project: CO2 hydrates — Challenges

and possibilities). The authors wish to thank Ing. Alain Valtz for his technical

assistance and fruitful discussion.

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